The present invention relates to electromagnetic surveillance systems, and more particularly to an apparatus for analyzing the signals received thereby emitted from articles within the system in response to the driving field.
A wide variety of electromagnetic surveillance systems has been developed in an attempt to deter pilferage from retail stores. Typically, these systems are positioned at the entrances and/or exits to retail stores so that individuals entering or leaving the store must pass through the system. The most common type of system includes a device for generating an electromagnetic field and an antenna for receiving signals emitted by articles within the system in response to the electromagnetic field. By analyzing the signals so received, it is possible to determine what types of articles are present within the system. Unauthorized articles include markers or tags, preferably in the nature of Permalloy or other material of high permeability, hidden or affixed to merchandise. If an article within the system is determined to be a Permalloy strip, the surveillance system sounds an alarm or otherwise alerts store personnel to the fact that pilferage is occurring. Examples of such surveillance systems are provided in U.S. Pat. No. 4,535,323, issued Aug. 13, 1985, entitled PREAMPLIFYING AND SIGNAL PROCESSING METHOD AND APPARATUS FOR THEFT DETECTION SYSTEMS; U.S. Pat. No. 4,524,350, issued June 18, 1985, entitled DETECTION LOGIC AND SIGNAL PROCESSING METHOD AND APPARATUS FOR THEFT DETECTION SYSTEMS; and U.S. patent application Ser. No. 364,264, filed Mar. 14, 1983, entitled INHIBIT AMPLIFIER AND PROCESSOR METHOD AND APPARATUS FOR THEFT DETECTION SYSTEMS, by the present inventors and/or co-workers, and owned by the assignee of the present application.
One problem occuring with prior systems is the less than 100 percent certainty in determining the types of articles moving through the surveillance system. Although the Permalloy strips have a rather distinctive response to the electromagnetic field generated by the system, their response is not unlike certain other permeable materials such as keys, pocket knives, pens, and other articles routinely carried by individuals. The uncertainty in identifying the Permalloy tags dictates one of two choices. First, the system can be "oversensitive" meaning that an alarm is sounded any time that an article which might conceivably be a Permalloy strip is moving through the system. The drawback of this approach is that the alarm will sound occasionally when an object other than a Permalloy strip is moving through the system, embarrassing the customer then moving through the surveillance system. This is extremely undesirable among store owners. Second, the system can be "undersensitive" meaning that the alarm is sounded only when the system is absolutely sure that a Permalloy tag is moving therethrough. The system can however miss tags which pass through certain areas of the opening. Pilferage will therefore continue even though the surveillance system is in place.
Accordingly, a significant drawback in prior art systems is the lack of sensitivity and/or selectivity of the signal analysis circuitry causing the system to sound erroneous alarm signals and miss Permalloy tags.